1. Field of the Invention
This invention relates to a cross-flow fan used for circulating gas in a gas laser device, and more particularly a cross-flow fan used in an ArF excimer laser device and a fluorine (F2) laser device.
2. Description of the Related Art
As semiconductor circuits are made smaller and more highly integrated there is a demand for projection exposure devices with an improved resolution. It has therefore been promoted to attain short wavelength exposure light emitted from an illumination source and, as the illumination source for semiconductor lithography, a gas laser device such as an excimer laser device or fluorine laser device and the like for emitting light having a shorter wavelength than the discharge wavelength of a prior art mercury lamp has been employed.
In the excimer laser device, laser gas composed of fluorine (F2), argon (Ar) and rare gases such as neon (Ne) acting as buffer gas is filled with several hundred kPa within a laser chamber, for example. In the fluorine laser device, laser gas composed of fluorine (F2) and rare gas such as helium (He) acting as buffer gas is similarly filled in the laser chamber with several hundred kPa. Further, there are provided a pair of main discharge electrodes. A high voltage is applied to the main discharge electrodes to generate discharge whereby the laser gas acting as laser medium is excited.
An output mirror and an optical system for narrowing the bandwidth and generating a narrow spectral width of the laser beam and stabilizing the wavelength of the main wavelength is arranged in front or rear of the laser chamber, respectively. A laser resonator is constituted by an output mirror and the optical system for narrowing the bandwidth. When the laser gas is excited the light emitted from the chamber is amplified by the laser resonator and then emitted from the output mirror as the laser beam.
In this case, the gas laser device acting as the light source for exposure is operated such that one light emitting operation is performed every time one discharge operation is carried out between the main discharge electrodes, and upon completion of this one-time exposure the laser medium between the electrodes becomes rather non-uniform regarding its constitution or density. Therefore, in order to start a subsequent discharge, fresh laser medium must be provided between the main electrodes. That is, when the discharge is started while the laser medium is non-uniform an arc discharge may easily be produced, a uniform glow discharge required for laser oscillation cannot be attained and the problem of an unstable output may occur.
In recent years, the repetition rate of the laser oscillation required in the laser device, i.e. the number of times of oscillation per unit of time, has become high and, in the device itself, it is necessary to replace the said gas rapidly.
In the prior art devices, a cross-flow fan has been used as a gas circulation fan in a gas laser device such as an excimer laser device and a fluorine laser device. In one example of these devices, the gas flow rate between the electrodes is about 10 m/s and the number of rotations per minute of the fan to attain the flow rate is about 1000 (rpm) in an excimer laser device for exposure with a repetition rate of 1 kHz, depending on the internal structure of the laser chamber (such as the distance between the electrodes and the position of the pre-ionization means).
In addition, in case of an excimer laser device having a repetition rate of 2 to 3 kHz, the gas flow rate that has to be provided between the electrodes is about 20 to 30 m/s and the number of rotations of the fan for attaining the flow rate has to be about 2000 to 3000 rpm.
Such a prior art device as described above was adapted to the required repetition rate merely by applying means for increasing the number of rotations of the fan. However, in recent years, higher repetition rates such as 4 kHz have become necessary, and a mere increase of the number of rotations of the fan as proposed in the prior art could not provide the desired results.
Therefore, there has been a strong need for developing a gas laser device for exposure satisfying the mentioned requirements.
It is an object of the present invention to provide means, so far not known in the prior art, capable of achieving a stable oscillation even in a gas laser device having a high repetition rate of 4 kHz or more.
In order to attain the said object, the gas laser device of the present invention is an excimer laser device with a repetition rate of 4 kHz or more which comprises a laser chamber having laser gas filled therein, a pair of main discharge electrodes arranged in the laser chamber, a cross-flow fan for circulating laser gas at least between the main discharge electrodes within the laser chamber, and a bearing structure for rotatably supporting the cross-flow fan, wherein a diameter of the cross-flow fan is 150 mm or less and a peripheral speed is 25.0 m/s or more.
Further, the aforesaid peripheral speed is preferably 27.0 rn/s or more.
In a preferred embodiment, the aforesaid bearing structure comprises a roller or ball bearing wherein the number of rotations is 4500 rpm or less, and further in the case that the bearing structure is a magnetic bearing, the number of rotations is 5000 rpm or less.
As described above, in order to perform a stable laser oscillation with a high repetition rate, it is necessary to quickly remove old gas remaining around the electrodes after completion of one discharge from the electrodes and to quickly provide new gas between the electrodes.
One of the methods for enhancing this capability may be to narrow the width of the electrode (the width in a direction where gas is circulated). Shortening of the width of the electrode results in narrowing the distance width in the glow discharge generated between the electrodes. In this case, the space for removing residual gas becomes small so that a high clearing rate becomes possible as well. However, the glow distance width is restricted by the desired optical output of the illumination source and its lifetime. Therefore, in the case of illumination sources of the exposure devices for semiconductor integrated circuits, the gap cannot be made unlimitedly short. It can be said that a lower limit of this numerical value is normally 3 to 4 mm.
Another method consists in increasing the gas circulating rate (gas velocity) between the electrodes. The technology related to this proposal is disclosed in Japanese Patent LaidOpen No. Hei 10-223955 corresponding to U.S. Pat. No. 5,771,258. This prior art provides a system in which the amount of gas bypassing the discharge region is restricted as much as possible, components preventing the flow in the gas circulating passage are eliminated as much as possible, and undesired disturbances of gas are eliminated to smoothen the flow. Although such a technology as above is of course necessary, the aim of this prior art is to eliminate some of the causes decreasing velocity but not to positively increase the flow speed. Further, the prior art aims at repetition rates of about 1 kHz, but high repetition rates of 4 kHz or more in the present invention require a new technology increasing the flow speed itself.
In view of the foregoing, the method for positively increasing the flow speed of the circulated gas consists in improving the characteristics of the cross-flow fan (a lateral flow fan). This cross-flow fan is normally used in xe2x80x9copen airxe2x80x9d such as found in an indoor air conditioner, and studies about the characteristics increasing the gas velocity in a high pressure closed container as found in a gas laser device such as an excimer laser device to which the present invention is directed have not sufficiently been performed.
The present inventor has found characteristics of fans capable of satisfying the condition of a high repetition rate of 4 kHz or more in the specific environment of gas laser device with a high pressure closed container such as an excimer laser device and a fluorine laser device and the like. More particularly, the present inventor has extensively investigated the relation between the fan diameter and the number of rotations of the fan.
As described above, the gas laser device of the present invention has a fan outer diameter of 150 mm or less and the peripheral speed of the fan is 25 m/s or more so that it can provide a laser device which can stand high repetition rates of 4 kHz or more. In particular, the present invention has found conditions that can be applied to an environment specific to gas lasers with high pressure closed containers employing a cross-flow fan used in the prior art in open air-conditions such as an air conditioner and has adapted its utilization to high repetition rates of 4 kHz or more, which was said to be impossible in the prior art.